Saturday, January 31, 2015

Yesterday afternoon my twitter feed became populated with claims of everything from great powder to free refills in the Wasatch. Is this really what it has come to?

I know things have been really really hard this January, but buck up. This is Utah not Colorado! We don't get excited about snow until we hit English measurement double digits, meaning 10 inches or more. We thumb our noses at metric measurement double digits, meaning 10 cm (4 inches), something that nobody even reached yesterday. I'm sure the 2–3 inches of snow that fell on the Park City side improved the skiing, but you are snow snobs. Don't stoop to such levels!

A few more ensemble members and the GFS have recently shifted to pulling the storm track further south and giving us some action Tuesday and Wednesday. Keep your fingers crossed that the low probability outcome discussed in the post from Thursday verifies. Sometimes Mother Nature does roll snake eyes.

Friday, January 30, 2015

If you are trying to avoid Seasonal Affective Disorder (SAD), today's weather is not for you. It's about as grey and drab as it can get. Our camera peering at the Wasatch Range from the University of Utah upper campus shows thick cirrostratus clouds, but further adding to the darkness is the valley smog/haze, evident on the right side of the photo.

Source: MesoWest

Here's a view looking at the Wasatch Range from the west showing the nicely layered atmosphere with the valley smog and cirrostratus deck separated by a clear layer aloft. I guess the place to be today is somewhere in that clear layer.

Thursday, January 29, 2015

If the forecast from the 0600 UTC GFS verifies, the central Wasatch will remain sandwiched between storms to our south or storms to our north over the next week, getting just what we can from brush-by precipitation events.

First we have the system moving into the southwest today and giving them precipitation through Saturday. We are expected to be on the northern edge of this system.

Once that system moves downstream, the we get brushed by systems to our north. Again, we're right on the edge of the action.

In situations like this, a slight shift in storm position can make a difference and I like to consult forecast ensembles to get an idea of range of possibilities. Let's start with the Short Range Ensemble Forecast System (SREF), from which we can examine the potential for precipitation as the system affecting the southwest brushes by us the next couple of days. There are 22 members of this ensemble, of which only 9 produce any precipitation at all at the Salt Lake City grid point, with two generating over 0.15".

A forecast like that suggests we may see a few valley showers and mountain snow showers tonight through early Saturday morning, but accumulations will probably be minimal. The odds of a few inches during this period in the upper Cottonwoods aren't zero, but they're pretty low (less than 10%).

For the next week, we can look at our ensemble of downscaled forecasts from the North American Ensemble Forecast System (NAEFS) for the 7-day period beginning yesterday afternoon and running through next Wednesday afternoon. As can be seen below, the probability of at least 6 inches of total snow accumulation in this period is fairly low, maxing out in the central Wasatch at about 60 or 70%. The odds of more than 24 inches isn't zero, but it's less than 10%. We would need one of those systems to shift southward for that to happen.

Here's another way to look at it. Below we've extracted the accumulated water equivalent (top) and snowfall (bottom) from the downscaled NAEFS ensemble members for Alta. The vast majority of the members are producing 8 inches or less of snow for the entire period. There is one member, from the Canadian Ensemble, that is very excited about a huge storm late in the period (this always seems to be the case with the Canadian Ensemble. The Canadians clearly love snow!).

So, looking at all these ensembles, the most likely forecast scenario for the next week (i.e., through Wednesday) in the central Wasatch is that we continue to see drier than average weather, with just a few periods of snow showers or snow as we are brushed by systems to the south or north. Although not zero, the odds of a major storm are low and would require a more direct hit than forecast by most of the ensemble members.

REI Talk Tonight

I'll be giving a talk on my book Secrets of the Greatest Snow on Earth, including a look at topics such as the real reasons why Utah snow is so great, where to find deep powder around the world, and the avalanche history of Little Cottonwood Canyon, at 7 PM tonight (Thursday) at the Salt Lake City REI (3285 E 3300 S). The talk is free and open to the public, although REI does request that you register by clicking here. My friends from Weller Book Works will be selling copies of the book if you haven't picked one up yet. I'll be happy to sign any copies you bring or buy at the event.

Wednesday, January 28, 2015

The highest elevations of Arizona, New Mexico, southern Utah, and southern Colorado will be getting some late Thursday through Saturday thanks to moisture streaming into the region from the south and east as shown in the integrated water vapor and 850-mb wind forecast for 0600 UTC 30 Jan (late Thursday).

This leads to the development of wide-spread precipitation across the region. as illustrated by the forecast below for 1800 UTC (1100 MST) Friday 30 January.

The northern Utah mountains are skunked in the forecast for that time and that is largely the case through the weekend, although we may see a few lingering snow showers today and then perhaps a stray snow shower or two through Friday. Although a few inches fell in the northern Wasatch last night (the central Wasatch go the shaft), it appears we will make it through January with only one major storm. What a pity.

Those of you considering a southern road trip should keep an eye on the forecasts. It's a complex system and the timing and intensity of snowfall will vary by location and I can't possibly hope to summarize that here!

Tuesday, January 27, 2015

Yesterday afternoon's sounding from the Salt Lake City International Airport recorded a 700-mb (~10,000 ft) temperature of 7.8ºC. This is the highest 700 mb temperature ever recorded between December 17 and March 21 and in line with what we typically observe in mid June. I've annotated the observation on the sounding climatology below to show how anomalous the temperature is for January. The red trace represents the highest 700-mb temperatures at each of the twice-daily sounding times during the year.

Source: SPC

Perhaps not surprisingly, Alta set a record high for the day of 58ºF. This beat their previous record of 43ºF for the date by 15ºF!

Source: NWS

That's a huge trouncing, but the previous 43ºF record for yesterday was the lowest maximum temperature record during January (tied with Jan 3rd). The record for the entire month of January remains 59ªF, set on January 12, 1996 and January 20, 2005 (the latter noted above). Thus, the max temperature at Alta yesterday was pretty damn impressive, but not an all-time record breaker for the month.

Valley showers and mountain snowshowers are on tap for today and tonight and possibly a bit tomorrow. This looks to be a sporadic, hit and miss event and the models continue to flounder around on accumulations, but continue to call for a modest event. Through late tomorrow, the 6Z NAM is going for about 3 inches at Alta, whereas the 6Z GFS is in there for 7.5 inches, with most of this falling this afternoon and tonight. I'll stick with 3–6 inches for a storm total as that seems to be in the heart of the distribution and I'm not sure how much to weight the new high-res GFS for mountain precipitation.

Monday, January 26, 2015

Consider yourself transported to a Bizzaro world, one in which the upper-level flow is from the east and you can ski in short sleeves in January.

Here's the view of the Bizzaro world this morning. The upper-level flow is what we call highly amplified, with a ridge over western North America and a deep trough over the eastern United States (Northeast skiers rejoice, dumpage is coming your way!). There is also a closed upper-level low off of Baja California that will provide us with more Bizzaro weather in the near future.

Here's this morning's sounding from the Salt Lake City airport. Note the deep easterly flow from 800–550 mb. It will be an unusually short flight if you are coming to Salt Lake City from Chicago today. Temperatures increase from 0ºC at the surface to 7.2ºC at 775 mb. At 700 mb, roughly 10,000 feet, the free atmosphere temperature was 5.2ºC.

Source: University of Wyoming

That's quite steamy for late January. The graph below shows the daily minimum (blue line), median (black line), and maximum (red line) 700-mb temperature in all available soundings taken at the Salt Lake City or Ogden airports since 1948. I've annotated with the green line this morning's 700-mb temperature. The record for the month of January is 7.2ºC (12Z Jan 20). However, from January 21 to 10 March the highest on record is 5.8ºC (Feb 1). So, we are very close to as hot as it gets at 700-mb for this time of year. In fact, this morning's 700-mb temperature is about the median for June 1st. If we had the June sun rather than the January sun, I'd be calling for a high at the airport of 73ºF.

Source: SPC

That closed low will move northward and bring a monsoon-like surge of moisture into the southwest that will spread into Utah tonight and tomorrow. That sounds exciting, but we're missing the surface heating of summer and the models can't seem to get their act together and converge on a solution. The 12Z NAM is going for only 0.11" of snow-water equivalent through 11 PM Wednesday, whereas the 6Z GFS is going for 0.8" (the 12Z run isn't available as I write this). Your guess is as good as mine. Hold a gun to my head and I'd go for 3-6" in upper Little Cottonwood by 11 PM Wed.

Sunday, January 25, 2015

Here are your four weekend updates from the Wasatch Range.1. State of the Snow

Much of the snow in the Wasatch has been affected by wind, sun, or humans, but incredibly there is some good settled powder to be had in sheltered areas on the north side of the compass despite it being almost 2 weeks since the last storm.

This time of year you can really see the power of the sun as the south aspects are baking and in some cases have lost their snow, whereas you can find powder on the north aspects.

2. Warmth

I noted a couple of days ago about the building ridge and the potential for very warm temperatures at 700-mb (10,000 feet) today (Sunday) and Monday. Well, the great warmup is underway and the GFS forecast 700-mb temperature is 3.5ºC for 5 PM this afternoon and 5.8ºC for 2 PM Monday afternoon. You can kiss more of that south aspect snow goodbye.

3. The January "Monsoon Surge"

Tropical moisture will be streaming into northern Utah and giving us some precipitation Tuesday and Wednesday. It's an odd pattern for January and the model forecasts still lack consistency with regards to potential storm totals. Brett called it a "refresher" in this morning's Utah Avalanche Center advisory and I'm good with that at this stage. We'll see how it comes together.

4. The Storm Killing Rex Block
There's no end to the storm-killing Rex Block (a.k.a. high-over-low block). Although we get a flirtation with tropical moisture Tuesday and Wednesday, it pops right back up at full strength thereafter. Below is the Euro forecasts for next weekend (ensemble mean left, high-res forecast right) and we're high-and-dry if that forecast verifies.

Source: ECMWF

A fairly similar forecast exists in the GFS. I've added one of our regional panels below for late Friday night to show the net impact of the Rex Block on precipitation. High and dry over the northern portion of the western U.S., with precipitation confined to the southwest.

How long could this go on? Well, let's just say that the overall pattern persists in both the 10-day GFS and ECMWF forecasts. Of course, there's always hope that the models are off the mark at those long lead times...

Friday, January 23, 2015

There is much to be learned about future climate change by looking at the present or the past and that certainly is the case for the current water year, which began on October 1st.

So far, it has been an unusually warm water year with an average temperature of 43.1ºF at the Salt Lake City airport through January 20. Only four similar periods in the instrumented period have been warmer.

One way to isolate the influence of temperature on the mountain snowpack is to examine the fraction of water year precipitation that is retained in the snowpack. This fraction will be near 100% if most of the precipitation falls as snow and there is little loss of snow on the ground to either melting or evaporation and it will be less than 100% (possibly far less) if some of the precipitation falls as rain and is not retained in the snowpack (for example, rain that falls before snow is on the ground or falls on a snowpack that is water saturated) or if there is melting or evaporation.

We can do this using data collected by USDA Natural Resources Conservation Service SNOTEL stations. These remote, automated stations measure water year precipitation using a large storage precipitation gauge (liquid precipitation equivalent - i.e., the combined water from rainfall and frozen precipitation after melt) and the amount of water in the snowpack (i.e., snow water equivalent) using a pillow that rests on the ground and measures the weight of the snow.

These observations are not perfect. Sometimes the precipitation gauge doesn't collect all the snow that is falling due to strong winds. Sometimes the snow depth on the pillow might be unrepresentative due to wind transport. Nevertheless, they are quite useful.

We begin with a low-elevation SNOTEL station characteristic of what I'll call a vulnerable snow climate, the Ben Lomond Trail SNOTEL at 5829 ft at the base of Ben Lomond Peak in the North Ogden Valley. This is the lowest SNOTEL in the Wasatch Mountains. This site has observed 10.1 inches of precipitation (black line) of which only 6.5 inches (64%, blue line) is retained in the snowpack.

Source: NRCS

Most of the 36% not in the snowpack is precipitation that appears to have fallen as rain and was not retained in the snowpack. There are only a couple of brief downturns where there is water loss due to melting (note: the snow on the surface can melt without net loss of snowpack if it refreezes as it percolates through the pack). This site is well shaded from the afternoon sun and thus tends to retain snow well. More sun exposed areas near this site likely saw greater losses at times.

Next, let's go up the mountain to an upper-elevation station characteristics of what I'll call a resilient snow climate, the Ben Lomond Peak SNOTEL at 8000 feet. This site has observed 15.5 inches of precipitation (black line) of which 14.1 inches (91%, blue line) is retained in the snowpack. Thus, despite the warmth of the water year, most of the precipitation that has fallen is still stored in the snowpack.

Source: NRCS

One can find similar results at other upper elevation sites, such as Thaynes Canyon at Park City Mountain Resort.

Source: NRCS

So, this has been a warm year and it has had an impact on the snowpack at lower elevations because a significant fraction of precipitation has fallen as rain. At the lowest elevations and on aspects that receive afternoon sun, there have also been losses due to melting. This is why the cross country center at Mountain Dell has sometimes looked less than desirable.

Mountain Dell, January 12, 2015

At upper elevations, however, most of the precipitation that has fallen (about 90% or more) is still retained in the snowpack on northerly aspects and other shady locations. These areas have some insurance against warming because of their aspect and their altitude. I emphasize northerly aspects and other shady locations because there clearly have been losses due to melt on other aspects this year (except perhaps at the highest elevations).

Overall, this provides some glimpse of the future snow climate of the Wasatch Mountains. It remains unclear how our average precipitation climate will shift in the future (the models have varying projections), but warming is coming. It is likely that the fraction of precipitation retained in the snowpack at low elevations during winter and at the end of the snow accumulation season will decline. Upper elevation northerly aspects have some insurance against the initial wave of global warming and their future will depend on just how high greenhouse gas concentrations get and how much the climate warms.

Thursday, January 22, 2015

A remarkably high amplitude upper-level pattern is setting up over the western United States, one that will bring record or near record temperatures aloft and then the possibility of an almost monsoon-like surge of tropical moisture into the southwest and eventually Utah. Yup, we are heading into outlier mode.

The loop below shows the latest forecast from the GFS and some of the key large-scale circulation features at play. Over the next couple of days, a high amplitude upper-level ridge develops along the west coast and progresses eastward. Nearly concurrently, a closed low forms off the coast of the Gulf of California. This is a result of what meteorologists call anticyclonic wave breaking and it results in a high-over-low block (a.k.a., Rex block) over the western United States. Subsequently, the closed low draws tropical moisture northward over the Gulf of California and into the southwest U.S. in a manner that is reminiscent of monsoon surges during the warm season.

The high-over-low pattern is most pronounced at 0000 UTC 26 Jan (late Sunday afternoon MST) with the ridge centered over Nevada and the trough centered west of Cabo.

We're very fortunate that we are starting this period with relatively little pollution and no snow over the valleys of northern Utah as temperatures aloft may be reaching record high values as the ridge moves overhead. Forecast 700-mb (10,000 ft) temperatures are around 3ºC Sunday afternoon and peak at 6ºC on Monday afternoon. If 6ºC verifies, and a sounding is taken at the time of the peak, it would be the highest 700-mb temperature in the Salt Lake City sounding record from late-Jan to early March.

Source: SPC

Although we have high confidence of the building ridge and warm temperatures aloft, the forecast after Monday gets a bit dicier. Let's talk first about the GFS forecast, which by Tuesday has the ridge axis to our east and tropical moisture streaming northward into Utah. The pattern reminds me a great deal of the monsoon surges we get during the warm season, although the easterly flow in this instance is displaced quite a bit further south of its typical location during the warm season.

Nevertheless, if this forecast verifies, the surge of moisture is going to bring some rain into the southwest. Although the southwest sorely needs precipitation, this is going to be a very warm storm and snow levels will probably be as high as 9000 or 10000 feet over Arizona, so additions to their mountain snowpack will be limited. The GFS then spreads moisture and showers into northern Utah late Tuesday and Wednesday.

The ECMWF model, however, has different ideas and has the moisture surge further west, so we'll have to see if and how forecasts come together the next few days. Personally, I'm hoping for a GFS-like solution. This winter has been so bizarre, we might as well continue the streak of weirdness.

Wednesday, January 21, 2015

Yesterday was remarkably clear and beautiful and I had the good fortune of traveling to Utah Valley University on the Frontrunner train to give a talk. This was my first time taking the Frontrunner south from Salt Lake Station and it is a great ride, especially when you can sit there gaping at big, white mountains the entire way. It has to be the most scenic commuter rail ride in America.

Many people asked me where the inversion was. It's such a shame that when we get a clear day this time of year we're surprised, but that's the way it is.

So where was the inversion yesterday? Too high to cause a major increase in pollution and smog. Yesterday afternoon's sounding shows the inversion (red shaded area) way the heck up there above 700 mb and generally above all but the highest peaks. Below the inversion, the atmosphere was fairly well mixed, so our pollution was dispersing nicely. In addition, we had some light northwesterly flow to help carry it away.

Overnight, the inversion has lowered, but even today we have a weak system dropping down from the north to weaken the inversion this afternoon and keep things stirred up. Enjoy the clear, clean air while it lasts.

Now, to answer a few of the questions posed in comments to yesterdays blog:

1. Why are there relatively few air quality sensors in Salt Lake and Utah Valleys?

The Utah Division of Air Quality maintains a number of air quality sensors in the Salt Lake and Utah Valleys, but the data is not available in real time. It is my understanding that they will be adding real-time capabilities at a couple more sites in the Salt Lake Valley in the near future. In my view, the historical lack of real-time sensors in the valley has reflected expense, bureaucratic barriers, and a lack of political will.

Of course, there is more to measuring and understanding air pollution than just measuring the PM2.5 concentration. It is critical to understand the constituents of that pollution and the processes involved in generating it. Perhaps I can find a guest blogger to address some of these issues and the efforts and costs involved for a future post as it is outside my area of expertise.

2. In Davis and Weber Counties, what holds the pollution in on the west
side of the counties. Does the inversion go all the way across the lake
to the mountains on the west side of the Salt Lake?

During most inversion events, the meteorological inversion, meaning the layer aloft in which temperature increases with height, extends across the Great Salt Lake and the entire Great Salt Lake Basin. There is really nothing that holds the pollution on the west side of the Great Salt Lake. In fact, at night, when the flow tends to be offshore (southeasterly over the Salt Lake Valley or easterly over the northern Wasatch Front), some of the pollution is transported from the urban area over the Great Salt Lake, only to return the next day as the lake breeze develops.

I don't know, however, how far west the pollution goes or, more specifically, what the PM2.5 levels get west of the lake and in rural northwest Utah during these events. Perhaps DAQ has looked into this. It is an important issue for air quality control strategies given the huge size of Utah counties.

Tuesday, January 20, 2015

If you want to ski fresh snow, you'd better call in sick and head up ASAP to enjoy the angry inch that fell in some locations last night. After that, the odds favor a mainly dry 7-day period as illustrated by the NAEFS forecasts below which show yesterday evening's brief flurry followed mainly by flatline forecasts (i.e., no precipitation).

The Canadian ensemble (CMCE) always seems to have at least one optimistic member, so you can always pin your hopes on that outlier (although I wouldn't).

I recently extended the GFS forecasts graphics on weather.utah.edu from 180 to 240 hours, so in situations like this you can look even farther into the future for a powder hit. Yeah, predictability is low at those long time horizons, but you can look anyway even though I call it "dream-prog land." Last night's forecasts show an almost monsoon-like pattern at 186 hours out with a warm surge of moisture pushing northward into the state. Perhaps it won't stay dry forever. Then again, this is a 186 hour forecast...

If there's anything you'd like me to talk about the next few days, leave a comment. I'm gonna be looking for ideas!

Announcements:

Wellers Book Works in Trolley Square has received a fresh batch of Secrets of the Greatest Snow on Earth. Stop by and pick up a copy if you are still looking for one.

I'll be speaking at noon today at Utah Valley University, Room SB260, about all things powder. It is open to the public, so stop by if you are in the area.

I'll also be giving a talk on Secrets of the Greatest Snow on Earth at the 3300 South REI at 7 PM on Thursday January 29th. The talk is free, but REI asks for advanced registration (click here to register).

Sunday, January 18, 2015

Model forecasts for the next 10 days are discouraging for powderhounds to say the least. Other than the potential for dribs and drabs generated by weak systems, there's pretty much nothing in the way of potential storms for northern Utah. Further, during the next week, a high-amplitude blocking ridge takes hold along the west coast. It's even in the ECMWF forecast!

Source: ECMWF

To illustrate how grim things are, the forecasts below are for the next week and have been derived from the North American Ensemble Forecast System (NAEFS) and downscaled for local terrain effects. The average of the members we are using calls for under 3 inches of snow over the next week.

And the corresponding model traces for Alta shows only one member getting to 6 inches and most with no more than two inches.

Yup, the situation is nearly hopeless. However, these are extended-range forecasts, so perhaps we can hope that we get a surprise. For instance, the ridge could shift upstream and open us up to northwesterly flow. While I can't rule out such a surprise, the odds are more than likely that we will see very little in the way of snow over the next week (maybe even 10 days). At least there's no snow on the valley floor, which should help some with the intensity of the inversion.

Saturday, January 17, 2015

I often tell people that a bad year in the Cottonwoods is better than a good year in Colorado. There are a couple of reasons for this. First, a bad year in the Cottonwoods is still probably 300-400" of snow (depending on location) which is more than average for much of Colorado. Second, Colorado is a beautiful state and has a lot going for it, but it's still fun to poke fun at them from time to time!

I was thinking today as I was ski touring on a gorgeous bluebird day that this really hasn't been a great ski season by Cottonwood standards, but we still have it good compared to many other regions. I've had some pretty good days this season and, even today, several days after the last big dump, there's still good snow to be found out there.

An old-school Wasatch ski tourer enjoys great powder and has the good sense to spoon his partner's track and conserve the hill for others that follow. Such powder conservation is especially important when storms are widely spaced.

Plus, it's only a short drive home to Salt Lake City from this great skiing. It always amazes me when I'm out skiing powder and I can see Salt Lake City only a few miles away.

Although by Cottonwood standards this is a fair year, this visiting Colorado ski tourer is all smiles because it's still better than he can find at home. And, he has no I-70 traffic jam tocontend with!

Here's another amazing thing. We were skiing dry powder at elevations down to 8000 feet, yet the high temperature at the University of Utah was 51ºF!

Friday, January 16, 2015

As discussed in an earlier post (see http://wasatchweatherweenies.blogspot.com/2015/01/upgrade-of-gfs.html), the GFS underwent a substantial upgrade on Wednesday of this week, including an increase in resolution to an effective grid spacing of about 13 km, very similar to the NAM.

In addition, for the first time, NCEP is kindly providing high-resolution vertical profiles and time series from the GFS for Alta (in what is known as BUFR format).

With help from Trevor Alcott, we've upgraded several of our products to take advantage of these improvements. For example, the Little Cottonwood guidance tables, GFS time-height sections for Alta, and GFS meteograms for Alta at weather.utah.edu are now based on the high-resolution BUFR output.

If you have been adjusting the old GFS precipitation forecasts to estimate precipitation at Alta (or other locations), you are probably going to need to tweak your approach some. At least at Alta and other mountain locations, you're probably going to see more precipitation generated by the GFS than you used to.

Just for kicks, here's what the GFS is generating for Alta over the next week for snow-water equivalent. A few dribs and drabs today and tonight, and then two small events on Sunday Night/Monday morning and then Thursday night/Friday morning. I suspect the old GFS would have produced less total precipitation due to its inferior resolution.

I'm hoping to eventually upgrade the GFS horizontal maps on weather.utah.edu to something based on higher resolution output grids. Unfortunately, NCEP is not providing output grids with anything near the full resolution of the new GFS, so while I can do better than we have now, we're still going to be looking at the model output at roughly half the full resolution of the GFS. What a bummer! To paraphrase Jeff Spicoli, "we upgraded the GFS 'cause it was bogus; so if we don't get some high-res output grids - pronto - we'll just be bogus too!"

The National Climatic Data Center released their end-of-year global analysis today concluding that 2014 was the warmest year in the instrumented record, with a globally averaged temperature 1.24ºF above the 20th century average and 0.07ºF warmer than the previous front runners 2005 and 2010 (see graph above). You can access their full summary here.

I look at these year-to-year records or non-records as a bit of distraction as ultimately it is the long-term trend that really matters and it is clearly upward. This upward trend is even more apparent when one takes a holistic view of the Earth's climate system (see Global Warming Hasn't Stopped). 2014 shouldn't get too excited about this record as it won't have it for very long.

Thursday, January 15, 2015

The loop below shows how Mother Nature is putting the squeeze on us. In the previous post we discussed how you can warm the air through subsidence or sinking motion and you can see that very well in the evolution of upper-level sounding profiles below which shows a strengthening and lowering of the inversion over the previous two days.

Yesterday afternoon low clouds covered much of the Salt Lake Valley and many of the other valleys and basins of northern Utah. Below shows the view from our mountain meteorology lab.

Although dismal and grey, I was actually hoping that those clouds would hang on. There is often a layer with good mixing beneath stratus and stratocumulus clouds of that type that is much deeper than we see during our typical inversion events. Indeed that was the case yesterday and infact there was a deep surface based mixed layer that extended to the clouds, enabling our pollution to mix through a layer that was about 3000 feet deep. That helps keep the pollution diluted.

Unfortunately, with those clouds gone now and I suspect we will see strengthening of the inversion at lower levels, with the mixing depth lowering and pollution concentrations increasing more rapidly. Plus, there's also reports of dense fog near the Great Salt Lake. Yes, Mother Nature is going to squeeze us further, although we have ourselves to blame for the pollution.

There is some good news. We will get clipped by a weak system late Friday and Friday night, so hopefully that will stir things up a bit and keep things from getting out of hand.

Announcement for Utah County readers:
I will be giving a talk at Utah Valley University (Building/Room SB 260) at noon next Tuesday, January 20, 2015 on my book Secrets of the Greatest Snow on Earth. The talk is open to the public and we are hoping (likely, but not firm yet) that there will be a book sales and signing event immediately following. Air Quality Links:

A number of you have asked where you can find air quality monitoring data. Here are the sites I use most frequently:

The University of Utah and Neil Armstrong Academy links above will provide data in tabular form. If you want a graph, click on "New Graphical Display" in the left hand bar and select "PM_2.5 Concentration" under one of the variable selectors. TRAX data is not always available since the car is not always in service.

Wednesday, January 14, 2015

Many people think that inversions form because air in the valley cools, and this can be a mechanism for inversion formation or strengthening at night, but the primary mechanism for the formation of the persistent inversions that plague us during the winter is the warming of air aloft following an incursion of cold air at low levels. This isolates a pool of cold air within the valley that is too dense to mix efficiently with the lower density air aloft. Given the limited heating by the sun from November to February, we're stuck with a cold airmass that gets increasingly polluted with time.

There are two major mechanisms that can contribute to the warming aloft. The first is the horizontal transport of warm air, something that meteorologists call horizontal temperature advection. The second mechanism, which can be very important but is often under appreciated, is the sinking of air aloft.

As air sinks, its pressure increases and it is warmed by compression at a known rate of 9.8 ºC/km (5.5 ºF/1000 ft). Meteorologists sometimes call this subsidence warming. Because the air originates at upper levels, it also tends to have low water vapor content and thus a low dewpoint.

This morning's sounding shows the tell-tale signs of subsidence warming. A relatively deep valley cold pool is surmounted by a 2ºC inversion from 750 to about 700 mb. Within the inversion layer, the dewpoint temperature drops dramatically and is very low (< -40ºC) at and just above the top of the inversion. This is usually a dead giveaway that the air at the top of the inversion air is of subsided origin.

However, we also have additional evidence of sinking air aloft in the computer model analyses. The 500-mb analysis for 0600 UTC, about 6 hours before the morning sounding was taken, shows that northern Utah was located beneath an area of strong sinking motion aloft in the wake of the 500-mb trough that gave us that great storm on Monday. The yellow contours (a bit hard to see) show the areas of sinking motion.

Due to a large degree to this subsidence, the inversion will both strengthen and lower over the next 24 hours, leaving us trapped in an increasingly shallow layer of cold, stable air. This inversion, however, may have some different characteristics than the last as it is very moist already, with wide spread low clouds. More on the significance of those clouds perhaps in a future post.

Monday, January 12, 2015

On Monday, it was almost as if Mother Nature had stored up two weeks of snow and decided to see if she could puke it out in just a few hours. Snowfall rates were high and sustained throughout the central Wasatch. I felt like a kid in a candy shop watching the automated snow depth sensors (about the best I can do when I can't be there). It was quite an event, and it exceeded all meteorological expectations.

I had the good fortune of not making a forecast for this event and I can tell you if I did I never would have gone for 16 inches (and 1.28" of water) in 10 8 hours (from 9 am to 5 pm) as was observed at Alta-Collins. Official forecasts called for a 70% chance of 3-6" in the 5 am to 5 pm period, and a 30% chance of 7-9". Note that they were calling for more after 5 PM, so overall, this was expected to be a pretty big event, but the early onset and intensity of the snowfall on Monday were not anticipated.

Source: NWS

The NWS forecast is specific with regards to time. Although I couldn't find other forecasts online specific for the 5 am to 5 pm time period, I did find that most issued the night prior to the storm were calling for storm totals of no more than 15". I might have gone for 4-8" in the 5 am to 5 pm period. I am 100% certain I would not have gone for a foot by 5 PM.

Such high intensity events are very hard to forecast, yet they are very important for public safety. Avalanche danger escalates rapidly in high intensity storms, as occurred on Monday, and highway maintenance is far more difficult.

We have a saying in meteorology, "live by the models, die by the models" and in this instance, none of the models were calling for such high snowfall rates. Even the 4-km NAM, well known for overforecasting, was going for a storm total of about 9 inches through Tuesday morning and didn't start the precipitation in earnest until Monday afternoon when it started at 9 am.

Thus, we clearly have more work to do, but you probably knew that already.

I do, however, have a forecast that is sure to verify. Unless we reach "too much of a good thing" accumulations tonight, you won't be seeing a new post on this blog Tuesday morning.

I don't know what kind of voodoo you people have been doing, but boy it sure looks to be working.

Here's the 84-hour GFS forecast valid for 5 PM this afternoon. The upper-level tough is overs southern Nevada and California and the northern Wasatch misses out on most of the action.

Here's the 12-hour GFS forecast valid for 5 PM this afternoon. The trough is instead along the Idaho-Nevada boarder and we are getting walloped!

It's already deluging in the valley and nuking in the mountains. Our automated sensor in Albion Basin has recorded 0.38 inches of water equivalent and about five inches of snow in the past two hours, including an increase of about four inches from 9 to 10 AM.

With the start of the semester today, I'm unable to put together a proper forecast, but it looks like we're finally going to get our first big dump since Christmas week. Check out utahskiweather.com for the gory details.

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The Wasatch Weather Weenies discuss the weather and climate of the Wasatch Front and Mountains, western United States, and beyond.

Participants include aspiring and old-school atmospheric scientists, weather enthusiasts, powder snobs, and poor souls enrolled in classes taught by University of Utah Atmospheric Sciences Professor Jim Steenburgh. Many posts feature content or insights enabled by the support of the National Science Foundation, Office of Naval Research, and the NOAA/National Weather Service.

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